Visual
system

The visual system in humans allows individuals to assimilate
information from the environment. The act of seeing starts when the
lens of the
eye focuses an image of its surroundings onto a light-sensitive
membrane in the back of the eye, called the retina. The retina is actually part of the brain that is isolated to serve as
a transducer for the
conversion of patterns of light into neuronal signals. The lens of
the eye focuses light on the photoreceptive
cells of the retina, which detect the photons of light and respond by producing neural
impulses. These signals are processed in a hierarchical fashion by different parts of
the brain, from the retina to the lateral geniculate nucleus,
to the primary and secondary visual cortex of the brain. Signals from the retina can also travel
directly from the retina to the Superior colliculus.

Study
of visual perception

The major problem in visual perception is that what people see
is not simply a translation of retinal stimuli (i.e., the image on
the retina). Thus people interested in perception have long
struggled to explain what visual processing does to create what we
actually see.

Early studies on visual
perception

There were two major ancient Greek schools, providing a
primitive explanation of how vision is carried out in the body.

The first was the "emission theory" which
maintained that vision occurs when rays emanate from the eyes and
are intercepted by visual objects. If we saw an object directly it
was by 'means of rays' coming out of the eyes and again falling on
the object. A refracted image was, however, seen by 'means of rays'
as well, which came out of the eyes, traversed through the air, and
after refraction, fell on the visible object which was sighted as
the result of the movement of the rays from the eye. This theory
was championed by scholars like Euclid and Ptolemy and their followers.

The second school advocated the so called 'intromission'
approach which sees vision as coming from something entering the
eyes representative of the object. With its main propagators Aristotle, Galen and their followers, this theory seems to
have some contact with modern theories of what vision really is,
but it remained only a speculation lacking any experimental
foundation.

Both schools of thought relied upon the principle that "like is
only known by like," and thus upon the notion that the eye was
composed of some "internal fire" which interacted with the
"external fire" of visible light and made vision possible. Plato makes this assertion in his
dialogue, Timaeus; as does
Aristotle, in his De
Sensu.[1]

Leonardo DaVinci: The eye has a central
line and everything that reaches the eye through this central line
can be seen distinctly.

.

Ibn al-Haytham (also known as
Alhacen or Alhazen), the "father of optics", was the first to resolve
this argument, by refining intromission theory into what is now the
modern accepted theory of vision in his influential Book of
Optics (1021). He argued that vision is due to light from
objects entering the eye. However, he maintained that the part of
the eye responsive to light was the lens, whereas we now know it
was the retina. He developed an early scientific method emphasizing
extensive experimentation. He pioneered the scientific study of the
psychology of visual perception, being the first scientist to argue
that vision occurs in the brain, rather than the eyes. He pointed
out that personal experience has an effect on what people see and
how they see, and that vision and perception are subjective. He
explained possible errors in vision in detail, and as an example,
describes how a small child with less experience may have more
difficulty interpreting what he/she sees. He also gives an example
of an adult that can make mistakes in vision because of how one's
experience suggests that they are seeing one thing, when they are
really seeing something else. This can be easily related to the
famous saying "beauty lies in the eye of the beholder".[2]
Al-Haytham carried out many investigations and experiments on visual
perception, extended the work of Ptolemy on binocular vision, and commented on the
anatomical works of Galen.[3][4]

Leonardo DaVinci (1452–1519) was the first
to recognize the special optical qualities of the eye. He wrote
"The function of the human eye ... was described by a large number
of authors in a certain way. But I found it to be completely
different." His main experimental finding was that there is only a
distinct and clear vision at the line of sight, the optical line
that ends at the fovea. Although he did not use these words
literally he actually is the father of the modern distinction
between foveal and peripheral vision.

Unconscious inference

Hermann von Helmholtz is often
credited with the first study of visual perception in modern times.
Helmholtz examined the human eye and concluded that it was,
optically, rather poor. The poor quality information gathered via
the eye seemed to him to make vision impossible. He therefore
concluded that vision could only be the result of some form of
unconscious inferences: a matter of making assumptions and
conclusions from incomplete data, based on previous
experiences.

The study of visual illusions
(cases when the inference process goes wrong) has yielded much
insight into what sort of assumptions the visual system makes.

Another type of the unconscious inference hypothesis (based on
probabilities) has recently been revived in so-called Bayesian
studies of visual perception. Proponents of this approach consider
that the visual system performs some form of Bayesian
inference to derive a perception from sensory data. Models
based on this idea have been used to describe various visual
subsystems, such as the perception of motion or the perception of
depth.[6][7]

Gestalt
theory

Gestalt
psychologists working primarily in the 1930s and 1940s raised
many of the research questions that are studied by vision
scientists today.

The Gestalt Laws of Organization have guided
the study of how people perceive visual components as organized
patterns or wholes, instead of many different parts. Gestalt is a
German word that translates to "configuration or pattern".
According to this theory, there are six main factors that determine
how we group things according to visual perception: Proximity,
Similarity, Closure, Symmetry, Common fate and Continuity.

One of the reasons why Gestalt laws are often disregarded by
cognitive psychologists is their inability to explain the nature of
peripheral vision. In Gestalt theory, visual perception only takes
place during fixations.

However, during fixations both the high definition foveal vision
at the fixation point and
the peripheral vision are functioning.
Because of its lack of acuity and relative independence of eye
position (due to its extreme wide angle), human vision is an image
compressing system.

While foveal vision is very
slow (from only three to four high-quality telescopic images per
second), peripheral vision is very inaccurate but also very fast
(up to 90 images per second - permitting one to see the flicker of
the European 50Hz TV images).
Elements of the visual field are thus grouped automatically
according to laws like Proximity, Similarity, Closure, Symmetry,
Common fate and Continuity.

Analysis of eye movement

During the 1960s, technical development permitted the continuous
registration of eye movement during reading[8] in
picture viewing [9] and
later in visual problem solving [10] and
when headset-cameras became available, also during driving.[11]

The picture to the left shows what may happen during the first
two seconds of visual inspection. While the background is out of
focus, representing the peripheral vision, the first eye
movement goes to the boots of the man (just because they are very
near the starting fixation and have a reasonable contrast).

The following fixations jump from face to face. They might even
permit comparisons between faces.

It may be concluded that the icon face is a very
attractive search icon within the peripheral field of vision. The
foveal vision adds detailed
information to the peripheral first impression.

The cognitive and
computational approaches

The major problem with the Gestalt laws (and the Gestalt school
generally) is that they are descriptive not
explanatory. For example, one cannot explain how humans
see continuous contours by simply stating that the brain "prefers
good continuity". Computational models of
vision have had more success in explaining visual phenomena and
have largely superseded Gestalt theory. More recently, the
computational models of visual perception have been developed for
Virtual Reality systems - these are closer to real life situation
as they account for motion and activities which populate the real
world.[12]
Regarding Gestalt influence on the study of visual perception,
Bruce, Green & Georgeson conclude:

"The physiological theory of the Gestaltists has fallen by
the wayside, leaving us with a set of descriptive principles, but
without a model of perceptual processing. Indeed, some of their
"laws" of perceptual organisation today sound vague and inadequate.
What is meant by a "good" or "simple" shape, for example?"[13]

In the 1970s David Marr developed a
multi-level theory of vision, which analysed the process of vision
at different levels of abstraction. In order to focus on the
understanding of specific problems in vision, he identified (with
Tomaso Poggio)
three levels of analysis: the computational,
algorithmic and implementational levels.

The computational level addresses, at a high level of
abstraction, the problems that the visual system must overcome. The
algorithmic level attempts to identify the strategy that
may be used to solve these problems. Finally, the
implementational level attempts to explain how these
problems are overcome in terms of the actual neural activity
necessary.

Marr suggested that it is possible to investigate vision at any
of these levels independently. Marr described vision as proceeding
from a two-dimensional visual array (on the retina) to a
three-dimensional description of the world as output. His stages of
vision include:

a 2D or primal sketch of the
scene, based on feature extraction of fundamental components of the
scene, including edges, regions, etc. Note the similarity in
concept to a pencil sketch drawn quickly by an artist as an
impression.

a 2-1/2 D sketch of the scene, where textures
are acknowledged, etc. Note the similarity in concept to the stage
in drawing where an artist highlights or shades areas of a scene,
to provide depth.

a 3 D model, where the scene is visualized in
a continuous, 3-dimensional map.[14]

Contents

Study of visual perception

The major problem in visual perception is that what people see is not simply a translation of the image on the retina. Therefore it was difficult to explain what visual processing does to create what we actually see.

History

The breakthrough came with Ibn al-Haytham (Alhacen), the "father of optics". In his Book of Optics in the 1000s he argued that vision occurs in the brain, rather than the eyes. He pointed out that personal experience has an effect on what people see and how they see, and that vision and perception are subjective. He explained possible errors in vision in detail, and as an example, describes how a small child with less experience may have more difficulty interpreting what he/she sees. He also gives an example of an adult that can make mistakes in vision because of how one's experience suggests that he/she is seeing one thing, when he/she is really seeing something else.[1]